The transvinylation of carboxylic acids serves to produce vinyl esters. This is understood to mean the transfer of a vinyl unit of a reactant vinyl ester (1V) to a reactant carboxylic acid (2S) to generate a product vinyl ester (2V) and the corresponding acid of the reactant vinyl ester (1S).

The transvinylation of vinyl esters with carboxylic acids in the presence of palladium catalyst is known from EP 376075 B1, in which copper bromide and especially lithium compounds are used as cocatalysts.
In addition to palladium catalysts and mercury catalysts, ruthenium compounds are also used as catalyst in the prior art for transvinylation of vinyl esters with carboxylic acids. Ruthenium compounds are characterized by their high solubility, low volatility and high thermal stability. In addition, they have high, temperature-inducible activity. However, a problem in using ruthenium compounds as catalysts in the transvinylation of vinyl esters with carboxylic acids is the formation of carboxylic anhydride as a side reaction of the transvinylation. This side reaction reduces the selectivity of the reaction. In addition, the anhydrides of the reactant carboxylic acid are relatively high boiling by-products which can only be removed from the catalyst-containing reaction mass with considerable effort and therefore would accumulate on reuse thereof.
A process for transvinylation of carboxylic acids using various Ru compounds as catalyst is described in EP 351603 A2 (U.S. Pat. No. 4,981,973). In order to shift the equilibrium, it is recommended to remove one of the reaction products continuously from the reaction mixture. In the presence of water, increasing the concentration of catalyst is recommended. For instance, in Example 60 at a water content of 2%, a ten-fold higher catalyst concentration is used than in the anhydrous case. On completion of the transvinylation, the product mixture is separated by distillation. The recycling of the Ru catalyst is not described.
EP 497340 A2 (U.S. Pat. No. 5,210,207) describes a transvinylation process for preparing product vinyl esters whose boiling points are higher than that of the reactant vinyl esters. By reactive distillation of at least one of the product components, the reaction equilibrium is shifted to the product side. The distilled reactant vinyl ester is at the same time recycled into the reaction. The formation of anhydrides as by-product is described when using Ru catalysts. Their formation is favored by high reaction temperature, high degree of conversion, long residence time and by a high concentration of reactants. For maximum selectivity and minimizing anhydride formation the authors recommend the reaction to be conducted as far as possible at a low degree of conversion and low residence time. The reuse of an Ru-containing catalyst under such conditions is described in Example 3. Here, neodecanoic acid is transvinylated at a conversion of only 50%. However, the reactive distillation requires large excesses of reactant vinyl ester, which are necessary due to the short residence time, in order to provide sufficient reaction partner for the reactant carboxylic acid. High space-time yields are therefore not achievable with this process.
A process is described in WO 92/09554 A1 in which the reaction mass of reactant carboxylic acid and catalyst is separated in a first step after the transvinylation and the product vinyl ester is subsequently separated by azeotropic distillation. This process is especially aimed at the separation of acid/vinyl ester mixtures having small boiling point differences. The transvinylation itself is operated continuously. Only reactant vinyl ester and a mixture of catalyst and reactant carboxylic acid are continuously fed back to the reactor. The formation of anhydrides and also recycling thereof is not described.
On account of the problem already described when using Ru catalyst, the low selectivity due to anhydride formation, the process is predominantly operated using palladium catalysts in the specifically disclosed embodiments in the more recent publications of the prior art. WO 2011/139360 A1 describes a continuous process for preparing carboxylic vinyl esters by reactive distillation. In this case, vinyl acetate as reactant vinyl ester and the acetic acid resulting therefrom are distilled off continuously, wherein the vinyl acetate is recycled into the process. Exclusively Pd-catalyzed systems are given in the examples which are characterized by high selectivity and form no anhydrides. WO 2011/139361 A1 describes a very similar process, the only difference being that the transvinylation is not conducted continuously but semi-continuously.
WO 2013/117294 A1 describes a continuous process for preparing carboxylic vinyl esters. In contrast to the reactive distillation processes just discussed, the transition metal-catalyzed transvinylation is operated in the steady-state and the reaction mixture is separated in a subsequent step. WO 2013/117295 describes a further configuration of this process with a subsequent derivatization of the resulting conjugate acid of the reactant vinyl ester. In both documents, the low yield and selectivity of Ru-catalyzed transvinylations and a conversion-limited mode of operation for suppression of anhydride formation are referred to. In the Ru-catalyzed examples of both documents, in contrast to the Pd-catalyzed systems, low selectivities and subsequent low space-time yields are described despite low conversions.
The use of Ru catalysts in the transvinylation reaction has distinct advantages compared to Pd catalysts with respect to solubility, volatility, thermal stability and thermally inducible activity. The significant occurrence of anhydride formation, in contrast to Pd catalysis, is described in the prior art as a major disadvantage of these systems, which significantly degrades the selectivity of the process and thus the possibility of reusing the catalyst. Last but not least, distillative work-up of the product vinyl ester also favors the formation of anhydrides, since high temperatures, long residence times and high concentrations arise in the distillation bottoms. Even at an equimolar composition or low excesses of one of the starting components and thus low theoretically achievable conversions, anhydride formation cannot in this case be completely avoided. Such compositions would be of interest, however, since the mass flows could be minimized and high space-time yields could be obtained thereby. The reactive distillation processes described require a significant excess of reactant vinyl ester since this evaporates after only a very short residence time and is no longer available for the reaction. High space-time yields are therefore not achievable with such processes.